1. Field of the Invention
The present invention relates to an apparatus for processing semiconductor substrates in a furnace tube or a reaction tube in order to perform various kinds of processing of the semiconductor substrates, such as heat treatment, chemical vapor deposition (CVD), and liquid-phase epitaxy.
2. Description of the Prior Art
It is well known that when various treatments such as heat treatment, CVD and liquid-phase epitaxy are effected on semiconductor substrates, a tube made of a refractory material such as quartz is used. The tube is generally called as a furnace tube or a reaction tube. For brevity and simplicity, these tubes are hereinafter referred to as "furnace tube". The furnace tube in which a gas flows is unsafe if it is rotated. Accordingly, the furnace tube is commonly used in a fixed. non-rotatable condition. In the non-rotatable furnace tube, an ampoule tube or a boat is rotated within the furnace tube when required in the liquid-phase epitaxy.
When performing the heat treatment of semiconductor substrates, the semiconductor substrates held on a holder or boat are heated at high temperatures in the range of 100.degree.- 1100.degree. C. within a fixed, non-rotatable quartz furnace tube. Partly due to the thermal deformation of a wall of the quartz furnace tube, and partly due to the weight of the semiconductor substrates and boat, the furnace tube is curved or bent downward in the middle of the total length while it is used repeatedly. When the deformation of the furnace tube becomes excessive, the boat cannot be removed from the furnace tube. In order to prevent the foregoing problem, the furnace tube is manually turned about the axis through an angle of 180 degrees to change the top and bottom of the furnace tube at predetermined intervals of use. Even with this manual inversion, the furnace tube is eventually deformed as a result of use for about six months and, therefore, must be repaired.
Furthermore, the fixed, non-rotatable furnace tube has a drawback that radial temperature uniformity is difficult to achieve. In view of this drawback, a heater composed of separated upper and lower heater elements is used to adequately control the temperature distribution in the furnace tube. As an alternative attempt, a controller is used to control the flow of a gas introduced into the furnace tube. In spite of these attempts, radial temperature uniformity has not been achieved.
In the liquid-phase epitaxy, a semiconductor substrate holder or cassette is held within the furnace tube by means of a quartz cantilever (metals cannot be used due to contamination) and the cassette or an ampoule containing the cassette is rotated. In this instance, if the cassette or the ampoule is held in contact with the inside surface of the furnace tube during rotation, no load is exerted on the quartz cantilever, and hence, the quartz cantilever is free from deformation. With this holding arrangement, however, a high speed rotation of the cassette or ampoule is not possible. On the other hand, if the cassette or the ampoule is spaced from the inside surface of the furnace tube during rotation, a high speed rotation is possible. However, a heavy load is exerted on the quartz cantilever, tending to deform and sometime damage or otherwise break the quartz cantilever.